Electrochemical investigation into the interaction between various pyrrole moieties and the well-known electron acceptor, tetracyanoethylene

Abstract

The charge transfer (CT) complexes between tetracyanoethylene (TCNE) and various pyrrole moieties (pyrrole, N-methylpyrrole and N-phenylpyrrole) have been characterised by UV–vis spectroscopy with values for their extinction coefficients ( ε), equilibrium constants ( K), λ max values, heats of formation (Δ H), along with thermodynamic and kinetic information regarding their instability in solution being obtained. The tricyanovinylated products of the reaction between TCNE and the pyrrole moieties have been isolated and fully characterised. The three tri-cyanovinylated pyrrole species namely, 2-tricyanovinyl-pyrrole (C 4H 4N–C 5N 3), 2-tricyanovinyl- N-methylpyrrole (C 5H 6N–C 5N 3) and 2-tricyanovinyl- N-phenylpyrrole (C 10H 8N–C 5N 3), have been characterised electrochemically by cyclic and normal pulse voltammetries. Each compound exhibits an irreversible anodic wave at more positive potentials than the unsubstituted monomer species along with a reversible reduction based one-electron redox couple. The calculated diffusion coefficients are similar to solution phase species, C 4H 4N–C 5N 3 (3.77×10 −5 cm 2 s −1), C 5H 6N−C 5N 3 (1.099×10 −5 cm 2 s −1) and C 10H 8N–C 5N 3 (2.382×10 −5 cm 2 s −1). The reduction based redox processes for all synthesised compounds exhibited facile kinetics with heterogeneous rate constants, k 0, of 2.65×10 −2 cm s −1 (C 4H 4N–C 5N 3), 1.35×10 −2 cm s −1 (C 5H 6N–C 5N 3) and 7.85×10 −3 cm s −1 (C 10H 8N–C 5N 3). Conducting polypyrrole and poly( N-methylpyrrole) films were grown electrochemically, by both chronocoulometry and cyclic voltammetry from reaction solutions of TCNE and the various pyrrole moieties. However, it was found that it was possible to form conducting polymer films from acetonitrile solutions containing the monomer species (0.1 M) in the absence of background electrolyte and TCNE. Upon electrochemical switching in various electrolyte solutions, between oxidised and reduced states the polypyrrole films exhibited cation movement as the predominant process that maintains charge neutrality. The latter films were unstable towards electrochemical cycling with a loss of global electroactivity after ten scans. On the other hand the poly( N-methylpyrrole) films exhibited anion movement as the predominant process upon redox switching with the films exhibiting a high level of stability towards electrochemical cycling.